Liquid phase epitaxy slider/stator assembly having non-wetting growth well liners

ABSTRACT

A slider/stator device for the growth of thin film material, such as mercury cadmium telluride, in which the substrate, thin film and growth solution are kept free from impurities by use of sleeves, made, for example, from sapphire, in order to prevent contamination and adhesion of the growth solution to the slider assembly.

This application is a continuation of copending Ser. No. 664,493 filedDec. 13, 1983, now abandoned; which in turn is a continuation ofcopending Ser. No. 361,706, filed Mar. 25, 1982, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates generally to devices used in the growth ofsemiconductor material by the process of liquid phase epitaxy, and, moreparticularly, to slider/stator devices used in the growth of mercurycadmium telluride (HgCdTe).

HgCdTe is a semiconductor material similar to silicon, germanium andgallium arsenide. It is extremely useful as a sensing material inthermal imaging systems. One distinguishing feature of HgCdTe is that aslight variation of the relative proportions of mercury and cadmiumchanges the spectral sensitivity of the material. Although this can be agreat advantage in designing infrared detectors, it means that thequantity of impurities in the material must be strictly controlled,since they may also alter the electrical properties of the material.

Liquid phase epitaxy (LPE) is a common method for growing high purityHgCdTe. In LPE, a molten solution of mercury and cadmium dissolved intellurium is placed in contact with a cadmium telluride (CdTe)crystalline substrate. By reducing the temperature, mercury, cadmium andtellurium are forced to precipitate, forming a thin film HgCdTe crystalon the CdTe substrate. One of the most common techniques for growing LPEHgCdTe uses a sliding assembly to bring the molten growth solution incontact with the CdTe substrate.

In the slider technique, a substrate is placed in a recessed well in thetop surface of the stator, or base, of the device, such that thesurfaces of the stator and substrate are substantially co-planar. Theslider, which is positioned on the stator, includes a well which holdsthe molten growth solution. Typically, this well extends through theslider so that the growth material may be loaded through the top of theslider. In the proper configuration for film growth, the slider ispositioned so that the bottom of the growth solution well is in closeproximity with, but does not touch, the substrate. The entire assemblyis heated until the growth material is at the required temperature, andthen the slider is moved so that the well holding the growth solution ispositioned above the substrate. When the temperature is dropped, theHgCdTe precipitates as a thin film on the substrate. When the crystalgrowth is complete, the slider is retracted so that the growth solutionis no longer in contact with the substrate and the thin film, and theentire assembly is cooled to room temperature. Although the techniqueappears uncomplicated, there are several problems which reduce thequality of the thin film grown. In the past, both slider and stator havebeen made of graphite, a refractory material which is both chemicallynonreactive and easily machinable. Typically, the graphite which is usedhas been treated, impregnated with a hydrocarbon pitch and cured toreduce porosity and improve mechanical properties. It is then heated toa high temperature to drive off impurities and improve electricalproperties.

Despite these improvements, the design and composition of the sliderstill cause degradation in the film quality and yield. First, thegraphite surfaces of the slider flake and crumble into the growthsolution. These particles adhere to the film and reduce performance.Second, impurities in the graphite tend to leach into the growthsolution at high temperatures, resulting in contamination which altersthe electrical properties of the material. Finally, because thetellurium-rich melt solution wets graphite, it tends to seep into anygaps between the stator and substrate, effectively welding the finishedthin films and substrate to the stator.

Excessive film growth at the interface of the substrate and the growthwell interface has been noticed in LPE growth of gallium aluminumarsenide using the slider technique. This growth is the result ofthermally induced convection currents in the growth solution which areproduced when the slider assembly is cooled to force film precipitation.This problem has been solved by lining the growth solution well with aninsulating material, for example, sapphire, which reduces convectionwithin the melt solution. For example, see the paper by Tamargo, M. C.and Reynolds, C. L., entitled "Use of Sapphire Liners to Eliminate EdgeGrowth in LPE (Al,Ga)As, Journal of Crystal Growth, Vol. 55, pp. 325-329(1981), published by North-Holland Publishing Company. Althoughexcessive edge growth does not occur in the growth of LPE HgCdTe, it hasbeen found by the present invention that sapphire can be used in thedesign of the slider and stator to eliminate the design andcontamination problems described above.

It is accordingly a primary object of the present invention to providean improved apparatus for the growth of thin film semiconductors,particularly HgCdTe, by use of a slider/stator technique.

SUMMARY OF THE INVENTION

The above and other objects of the present invention are achieved bylining substantially all surfaces of the slider/stator assembly whichcontact the growth solution with a dense, pure, nonwetting material. Ina preferred embodiment this material is monocrystalline sapphire. Thegrowth well of the slider is lined with a sapphire sleeve and cap.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects of the present invention are achieved in the illustrativeembodiment as described with respect to the Figures in which:

FIG. 1 shows an isometric view of a slider assembly according to thepresent invention;

FIGS. 2A, 2B and 2C show cross-sectional side views on the subjectinvention during successive stages of LPE thin film manufacture; and

FIGS. 3 and 4 show the use of interchangeable growth wells which may beused in the slider assembly of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to FIGS. 1 and 2, the device of the present invention includea base, or stator 11, and sliding assembly 10. The slider 10 can bemoved along the length of stator 11, by virtue of a matched set ofgrooves 19 and rails 20 included in the stator 11 and slider 10,respectively. Two positions of the slider 10 are shown in FIGS. 2A and2B. Movement of the slider can be controlled by a pushrod, by tiltingthe assembly, or some other external means. All surfaces of the assemblywhich might contact or provide a source of particular contamination tothe growth solution 17 are lined with a dense, pure, refactory andnonwetting material, such as monocrystalline sapphire.

The stator 11 and slider 10 are machined from solid blocks of graphite.The overall size of the stator and slider is determined by the size ofthe desired thin film and associated substrate 12. Substrate spacers 13,well cap 16, stator cover 14, slider liner plate 18, and growth solutionwell liner 15 are made from planks or tubes of monocrystalline sapphire.In an alternate embodiment, the above-noted parts made from sapphiremight instead be made from polycrystalline alumina. All graphite andsapphire parts are purified before use by baking at high temperatures,for example, 1000 degrees Centigrade, under low pressures, for example,ten to the minus six torr, in a chemically inert environment. Allsapphire surfaces are lapped smooth to about 100 microinches. Graphitesurfaces are rough milled and mated through use.

The stator 11 is machined with two abutting rectangular wells. One wellis used to hold the thin film substrate 12 with associated spacers 13,and the other well includes stator cover plate 14. Typically, the wellfor the stator cover 14 is as wide as possible in the stator to keep itfrom sliding and also to help insure that the growth solution is notcontaminated by graphite. It is also long enough to underlie the growthsolution well 15 for the throw, i.e., alternate positions, of the slider10. The depth of the well for the stator cover plate 14 is made to matchthe thickness of the stator cover plate 14, such that the top surface ofthe plate 14 and the stator 11 are coplanar. The actual thickness of theplate is not critical. The well for the thin film substrate 12 ismachined to be slightly larger than the size of the desired thin film.Sapphire spacers 13 are cut to surround the substrate 12 within thewell. The depth of the well matches the thickness of the availablesubstrate, for example, 0.040 inches deep. The depth of the well forsubstrate 12 is also made so that the top surface of substrate 12 isslightly below the top surface of plate 14. Substrate spacers 13 may beas much as 0.010 inches thinner than the thickness of substrate 12without affecting performance of the device. Both wells for cover plate14 and substrate 12 may be end-milled to accept square-edged pieces. Theslider 10 includes a block of graphite which has been recess-milled toprovide minimal contact surfaces with the stator 11, as shown in FIGS.2A and 2B.

A well for the growth solution extends through the slider 10, and islined with a monocrystalline sapphire tube or liner 15. Thecross-sectional size and shape of the liner 15 is selected to match orto exceed the size of the desired finished thin film 22, for example,0.5 inches square. The thickness of the walls of the liner may beselected to enhance or diminish the insulating effect of the sapphireliner, as required. Typically, the liner walls are 0.030 inches thick.The base surface of the liner which faces the stator is fine ground orpolished flat to within 50 microinches to insure that the substrate 12or finished film 22 will not be scratched when the slider is moved. Theliner 15 is press fit into the slider such that the polished basesurface is within 0.001 inches of the top surface of the stator coverplate 14. In addition, a sapphire plank 18 may be press fit into therecessed base of slider 10 to prevent graphite flakes from dropping fromthe slider onto the thin film 22 or substrate 12. A sapphire cap 16 tothe growth solution well may be included to reduce contamination of thegrowth solution 17.

In an alternate embodiment, the section of the slider which includes thegrowth solution well is removable, and may be replaced with an insert 23for multiple or different sized substrates 24 as shown in FIGS. 3 and 4.In these configurations, the same slider and stator assembly may be usedfor a variety of thin film manufacturing tasks without building a newassembly for each new substrate design.

The subject invention operates in the following manner. Prior to use,all parts are assembled and purified at high temperatures as describedabove. The CdTe substrate 12 is placed in the substrate well of thestator 11 and surrounded on each side by sapphire spacers 13. The slideris positioned on the stator as shown in FIG. 2A; the HgCdTe growthmaterial 17 is loaded into the growth solution well, and then capped bycover 16. The assembly is placed in a reaction tube and furnace andbrought to the appropriate temperature. The slider is moved by pushrod,gravity or some external means so that the growth solution 17 is incontact with the substrate 12, as shown in FIG. 2B. The temperature ofthe apparatus is then lowered to force precipitation of the thin filmHgCdTe 22. After thin film growth, the slider 10 is returned to itsinitial position, as shown in FIG. 2C, so that the finished film 22 isremoved from contact with the growth solution 17. When the apparatus hascooled, the slider 10 may be removed, exposing the completed thin film22 on substrate 12. The substrate 12 with thin film 22 are then removedfor further processing for use in, for example, an infrared detectingdevice.

Having described the invention, what is claimed as new and novel and forwhich it is desired to secure Letters Patent is:
 1. Apparatus forproducing thin film materials, said apparatus comprising:A. a sliderassembly made out of a first material; B. a stator assembly made out ofsaid first material and having means for guiding movement of said sliderassembly on said stator assembly, said slider assembly sliding alongsaid stator assembly between a first position and second position; C.holding means, included in said slider assembly, for retaining a growthsolution, wherein said holding means is made of a second material and isused to prevent said growth material from contacting said first materialof said slider assembly; D. retention means, included in said statorassembly, for holding a thin film substrate material said retentionmeans being made of said second material; E. protection means mounted ona side of said stator assembly between said stator assembly and saidslider assembly and contiguous to said retention means, said protectionmeans being made of said second material; and F. prevention means madeof said second material contiguous with said holding means and mountedon a side of said slider assembly so that it is between said sliderassembly and said stator assembly, said protection means, saidprevention means, said retention means, and said holding meanscooperating to prevent said growth solution from contacting either thefirst material of said slider assembly or the first material of saidstator assembly as said slider assembly slides along said statorassembly between said first position and said second position to preventcontamination of said growth solution by said first material. 2.Apparatus as in claim 1 wherein said second material from which saidholding means, said retention means, said preventing means and saidprotection means are made is nonwetting so as to substantially preventcontamination and adhesion of said growth solution to said sliderassembly and to said stator assembly.
 3. Apparatus as in claim 2 whereinsaid second material is either sapphire or alumina.
 4. Apparatus as inclaim 2 wherein said holding means extends through said slider assemblyso that said growth solution may be placed in said holding means at oneend thereof while said slider assembly is on said stator assembly. 5.Apparatus as in claim 4 further comprising means for covering saidholding means at said one end after said growth solution has been placedin said holding means so as to substantially reduce contamination ofsaid growth solution.
 6. Apparatus as in claim 5 wherein said means forcovering may be made from either sapphire or alumina.